This invention relates to a neutron detector arranged, for example, in a nuclear reactor and particularly adapted to exactly detect neutrons in spite of lowering of insulation resistance of an insulating member constituting the neutron detector, the lowering of the insulation resistance being caused by a high temperature in the reactor.
Generally, neutrons are measured indirectly by detecting electrically charged particles or .gamma.-rays generated by the nuclear reaction of neutrons and atomic nucleus for the reason that the neutrons cannot be directly detected by ionization reaction because they have no electric charge. For this reason, a gas ionization chamber type neutron detector is used as a neutron detector in which a predetermined d.c. voltage is applied across an anode electrode and a cathode electrode disposed in the ionization chamber to generate an electric field therebetween. A neutron converting element, which reacts with the neutrons and converts them into electrically charged particles or .gamma.-rays, such as uranium, boron, or plutonium is baked on the surface of at least one of the anode and cathode electrodes. An inert gas such as argon or helium is charged in the ionization chamber and electrically charged particles generated by the reaction ionize the inert gas in the chamber to generate electrons and ions. Due to the generation of the electric field between the anode and the cathode electrodes, the ions and electrons are attracted to the anode and the cathode electrodes respectively thereby to pass an ionization current therebetween in proportion to the intensity of the injected neutron flux. Therefore, the injected neutron flux can be detected by measuring the ionization current thus generated.
However, in a case where a gas ionization chamber type neutron detector described above is disposed in a nuclear reactor under a high temperature environment, since specific resistance of an insulating member, such as alumina used to construct the ionization chamber is low under a high temperature environment, it is difficult to prevent the flow of leakage current which is proportional to the voltage applied across the anode and the cathode electrodes. In addition, the leakage current is added to the ionization current generated at the same time and this combined current is detected and measured as an output current. Therefore, it is impossible to obtain the true ionization current in proportion to the injected neutron flex by measuring the combined current. For example, even high purity alumina which is one of known inorganic insulation materials having the most highest stability with respect to heat becomes electroconductive at a high temperature of more than about 800.degree. C. and it cannot be used as an insulating material.
In order to obviate the defect described above and to use this type neutron detector for measuring the ionization current in proportion to the injected neutron flux, it has been desired to reduce the ratio of the leakage current to the ionization current to a negligible value i.e., 1/100 or less. The reduction of this ratio may be achieved by increasing a neutron sensitivity or by reducing the insulation resistance of the insulating material as much as possible. However, for increasing the neutron sensitivity, the dimensions of the ionization chamber must be enlarged, which is of course undesirable. Thus, in order to obtain actual ionization current created by the injected neutron flux, it is desired to suppress the tendency of lowering of the insulation resistance of the ionization chamber as much as possible.
FIG. 1 shows a vertical elevation of one of known gas ionization chamber type neutron detectors, in which an ionization chamber D is connected to the lower end of a guide cable C for deriving an ionization current out of the reactor core. At the substantially central portion of the ionization chamber is provided an anode electrode 1 and on the surface of a cathode electrode 2 facing the anode electrode 1 is deposited, by baking for example, a neutron converting element 3 consisting of at least one of uranium, boron, and plutonium which undergo a nuclear reaction with the injected neutron flux thereby to generate electrically charged particles. The cathode electrode 2 is constructed to act as an outer casing of the ionization chamber D. The anode electrode 1 is insulated from the cathode electrode 2 and supported by an inorganic insulating material 5 such as magnesia, alumina, boron nitride or silica, and an inert gas such as argon or helium is filled in a space between the anode and the cathode electrodes of the ionization chamber. The guide cable C comprises a central electric conductor 11 extending axially of the cable, an outer electric conductor 14 made of a metal coated tube arranged coaxially with the conductor 11, and an inorganic insulating material 15 such as alumina, magnesia, boron nitride, or silica filling the space between the electric conductors 11 and 14. The lower end of the central conductor 11 is electrically connected to the upper end of the anode electrode 1 and the lower end of the outer conductor 14 is electrically connected to the cathode electrode 2. The insides of the cable C and the ionization chamber D are air tightly sealed and separated by a partition wall 16 made of an inorganic insulating material such as magnesia, alumina, boron nitride or silica, and the upper end, not shown, of the cable C is also sealed in the same manner.
In a neutron detector described above, neutron flux injected into the ionization chamber undergoes nuclear reaction with only the neutron converting element 3 deposited on the inner surface of the cathode electrode 2 thereby to generate an ionization current which is measured through the conductor 11 by a known device disposed externally of the reactor core. However, since the interior of the reactor core is under high energy condition and high neutron flux density (about 10.sup.14 neutrons/cm.sup.2 /sec.), and since the reactor is operated at a high temperature of about 800.degree.-1000.degree. C., the insulation resistance of the insulating material constituting the neutron detector of the type described above is lowered and the leakage current is added to the ionization current, which makes difficult to measure only the actual ionization current created by the injected neutron flux.
An equivalent circuit of a neutron detector shown in FIG. 1 is shown in FIG. 2, in which currents I.sub.1, I.sub.2, and I.sub.3 flow through an insulation resistance R.sub.1 of the cable C, an insulation R.sub.2 of the partition wall 16, and the insulation resistance R.sub.3 of the inorganic insulating member 5, when a voltage is applied from a power source V. Current I.sub.0 corresponding to the sum of these currents I.sub.1, I.sub.2, I.sub.3 and an ionization current I.sub.4 created by the injected neutron flux passes through an ampere meter A. The equivalent circuit shown in FIG. 2 may be further simplified as shown in FIG. 3, in which current I.sub.0 corresponding to the sum of the ionization current I.sub.4 and current I.sub.R passing through an inner (anode) resistance R.sub.0 is measured by the ampere meter A. As can be understood from this circuit, the resistance R.sub.0 lowers when the inner temperature of the neutron detector increases and the current I.sub.0 also increases. Thus, the ampere meter A cannot indicate only the actual ionization current I.sub.4.